Involvement of AMPA receptors in the antidepressant-like effects of lithium in the mouse tail suspension test and forced swim test

Does increasing the ratio of AMPA-to-NMDA receptor mediated neurotransmission engender antidepressant action? Studies in the mouse forced swim and tail suspension tests

Monoamine-based antidepressant drugs increase α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) function and decrease N-methyl-d-aspartate receptor (NMDAR) function. The NMDAR antagonist ketamine shows potent antidepressant action in humans and the antidepressant-like effects of ketamine and monoamine-based antidepressants in rodents depend on increased AMPAR throughput. Further, the antidepressant-like effects of monoamine-based antidepressants are enhanced by AMPAR potentiation and by NMDAR antagonism. This has led to a hypothesis that antidepressant efficacy involves an increases ratio of AMPAR-to-NMDAR-mediated neurotransmission. To further elucidate the interaction of AMPAR, NMDAR and monoamine transmission we tested combinations of the AMPAR positive allosteric modulator (AMPA potentiator), (R,R)-N,N-(2,20-[biphenyl-4-40-diyl]bis[propane-2,1-diyl])dimethanesulfonamide (PIMSD), with: the uncompetitive NMDAR antagonist MK-801; nicotine, which has potent glutamate-releasing properties; and the selective serotonin reuptake inhibitor escitalopram using the mouse forced swim (mFST) and tail suspension tests (mTST). MK-801, nicotine or escitalopram did not induce antidepressant-like effects in either of the two tests. PIMSD enhanced the effect of MK-801 in the mFST, supporting the hypothesis that increasing AMPAR-to-NMDAR-mediated neurotransmission conveys antidepressant action. Nicotine-induced glutamate release simultaneously activates NMDARs and AMPARs and showed no net effect in the mFST when given alone. However, increasing the ratio of AMPAR-to-NMDA-R transmission by favouring AMPAR throughput with PIMSD revealed an antidepressant-like action of nicotine in the mFST. PIMSD also enhanced the effect of escitalopram treatment in the mFST and mTST, supporting existing evidence and suggesting a synergistic effect of simultaneously facilitating monoamine transmission and increasing the ratio of AMPAR-to-NMDAR throughput. No synergistic effects of the PIMSD+MK-801 or PIMSD+nicotine were found in the mTST, indicating a differential sensitivity of mFST and mTST when investigating glutamate-based antidepressant mechanisms. This study corroborates existing evidence that there may be an unexploited therapeutic potential in treating depression by directly increasing the ratio of AMPAR-to-NMDAR neurotransmission, possibly in combination with monoamine-based mechanisms.

GluA1 and its PDZ-interaction: a role in experience-dependent behavioral plasticity in the forced swim test

Glutamate receptor dependent synaptic plasticity plays an important role in the pathophysiology of depression. Hippocampal samples from clinically depressed patients display reduced mRNA levels for GluA1, a major subunit of AMPA receptors. Moreover, activation and synaptic incorporation of GluA1-containing AMPA receptors are required for the antidepressant-like effects of NMDA receptor antagonists. These findings argue that GluA1-dependent synaptic plasticity might be critically involved in the expression of depression. Using an animal model of depression, we demonstrate that global or hippocampus-selective deletion of GluA1 impairs expression of experience-dependent behavioral despair. This impairment is mediated by the interaction of GluA1 with PDZ-binding domain proteins, as deletion of the C-terminal leucine alone is sufficient to replicate the behavioral phenotype. Our results provide evidence for a significant role of hippocampal GluA1-containing AMPA receptors and their PDZ-interaction in experience-dependent expression of behavioral despair and link mechanisms of hippocampal synaptic plasticity with behavioral expression of depression.

Deletion of GSK3β in D2R-expressing neurons reveals distinct roles for β-arrestin signaling in antipsychotic and lithium action

Several studies in rodent models have shown that glycogen synthase kinase 3 β (GSK3β) plays an important role in the actions of antispychotics and mood stabilizers. Recently it was demonstrated that GSK3β through a β-arrestin2/protein kinase B (PKB or Akt)/protein phosphatase 2A (PP2A) signaling complex regulates dopamine (DA)- and lithium-sensitive behaviors and is required to mediate endophenotypes of mania and depression in rodents. We have previously shown that atypical antipsychotics antagonize DA D2 receptor (D2R)/β-arrestin2 interactions more efficaciously than G-protein-dependent signaling, whereas typical antipsychotics inhibit both pathways with similar efficacy. To elucidate the site of action of GSK3β in regulating DA- or lithium-sensitive behaviors, we generated conditional knockouts of GSK3β, where GSK3β was deleted in either DA D1- or D2-receptor-expressing neurons. We analyzed these mice for behaviors commonly used to test antipsychotic efficacy or behaviors that are sensitive to lithium treatment. Mice with deletion of GSK3β in D2 (D2GSK3β(-/-)) but not D1 (D1GSK3β(-/-)) neurons mimic antipsychotic action. However, haloperidol (HAL)-induced catalepsy was unchanged in either D2GSK3β(-/-) or D1GSK3β(-/-) mice compared with control mice. Interestingly, genetic stabilization of β-catenin, a downstream target of GSK3β, in D2 neurons did not affect any of the behaviors tested. Moreover, D2GSK3β(-/-) or D1GSK3β(-/-) mice showed similar responses to controls in the tail suspension test (TST) and dark-light emergence test, behaviors which were previously shown to be β-arrestin2- and GSK3β-dependent and sensitive to lithium treatment. Taken together these studies suggest that selective deletion of GSK3β but not stabilization of β-catenin in D2 neurons mimics antipsychotic action without affecting signaling pathways involved in catalepsy or certain mood-related behaviors.

Metabotropic glutamate 2/3 receptor antagonists improve behavioral and prefrontal dopaminergic alterations in the chronic corticosterone-induced depression model in mice

Metabotropic glutamate 2/3 (mGlu2/3) receptor antagonists have an antidepressant-like effect, but the exact mechanism still remains unclear. This study examined the effects of mGlu2/3 receptor antagonists in chronic corticosterone-treated mice which could be used as an animal model of depression. In the forced swim test, the mGlu2/3 receptor antagonists MGS0039 (1.0 mg/kg, i.p.) and LY341495 (0.3 mg/kg, i.p) significantly reduced the increased immobility time of mice pretreated with corticosterone (20 mg/kg, s.c.) for 21 days, while desipramine (30 mg/kg, i.p.) and fluoxetine (30 mg/kg, i.p.) did not. The antidepressant-like effect of LY341495 was not blocked by the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor antagonist NBQX (10 mg/kg, i.p.). Systemic administration of LY341495 did not affect basal release of glutamate, dopamine or serotonin in the prefrontal cortex of the control or chronic corticosterone-treated mice. Chronic corticosterone markedly enhanced high K(+)-induced release of dopamine, but not serotonin or glutamate, in the prefrontal cortex. This neurochemical change was blocked by systemic administration of MGS0039 and LY341495, but not desipramine or fluoxetine. These results suggest that chronic corticosterone-treated mice could be used as an animal model of treatment-resistant depression. This study also suggests that the prefrontal dopaminergic system is involved in the antidepressant-like effect of mGlu2/3 receptor antagonists in the chronic corticosterone-induced depression model.

Targeting the BH3-interacting domain death agonist to develop mechanistically unique antidepressants

The BH3-interacting domain death agonist (Bid) is a pro-apoptotic member of the B-cell lymphoma-2 (Bcl-2) protein family. Previous studies have shown that stress reduces levels of Bcl-2 in brain regions implicated in the pathophysiology of mood disorders, whereas antidepressants and mood stabilizers increase Bcl-2 levels. The Bcl-2 protein family has an essential role in cellular resilience as well as synaptic and neuronal plasticity and may influence mood and affective behaviors. This study inhibited Bid in mice using two pharmacological antagonists (BI-11A7 and BI-2A7); the selective serotonin reuptake inhibitor citalopram was used as a positive control. These agents were studied in several well-known rodent models of depression-the forced swim test (FST), the tail suspension test (TST), and the learned helplessness (LH) paradigm-as well as in the female urine sniffing test (FUST), a measure of sex-related reward-seeking behavior. Citalopram and BI-11A7 both significantly reduced immobility time in the FST and TST and attenuated escape latencies in mice that underwent the LH paradigm. In the FUST, both agents significantly improved duration of female urine sniffing in mice that had developed helplessness. LH induction increased the activation of apoptosis-inducing factor (AIF), a caspase-independent cell death constituent activated by Bid, and mitochondrial AIF expression was attenuated by chronic BI-11A7 infusion. Taken together, the results suggest that functional perturbation of apoptotic proteins such as Bid and, alternatively, enhancement of Bcl-2 function, is a putative strategy for developing novel therapeutics for mood disorders.

The tail suspension test

The tail-suspension test is a mouse behavioral test useful in the screening of potential antidepressant drugs, and assessing of other manipulations that are expected to affect depression related behaviors. Mice are suspended by their tails with tape, in such a position that it cannot escape or hold on to nearby surfaces. During this test, typically six minutes in duration, the resulting escape oriented behaviors are quantified. The tail-suspension test is a valuable tool in drug discovery for high-throughput screening of prospective antidepressant compounds. Here, we describe the details required for implementation of this test with additional emphasis on potential problems that may occur and how to avoid them. We also offer a solution to the tail climbing behavior, a common problem that renders this test useless in some mouse strains, such as the widely used C57BL/6. Specifically, we prevent tail climbing behaviors by passing mouse tails through a small plastic cylinder prior to suspension. Finally, we detail how to manually score the behaviors that are manifested in this test.

Combined treatment with the mood stabilizers lithium and valproate produces multiple beneficial effects in transgenic mouse models of Huntington's disease

Emerging evidence suggests that the mood stabilizers lithium and valproate (VPA) have broad neuroprotective and neurotrophic properties, and that these occur via inhibition of glycogen synthase kinase 3 (GSK-3) and histone deacetylases (HDACs), respectively. Huntington's disease (HD) is an inherited neurodegenerative disorder characterized by impaired movement, cognitive and psychiatric disturbances, and premature death. We treated N171-82Q and YAC128 mice, two mouse models of HD varying in genetic backgrounds and pathological progressions, with a diet containing therapeutic doses of lithium, VPA, or both. Untreated, these transgenic mice displayed a decrease in levels of GSK-3β serine 9 phosphorylation and histone H3 acetylation in the striatum and cerebral cortex around the onset of behavioral deficits, indicating a hyperactivity of GSK-3β and HDACs. Using multiple well-validated behavioral tests, we found that co-treatment with lithium and VPA more effectively alleviated spontaneous locomotor deficits and depressive-like behaviors in both models of HD mice. Furthermore, compared with monotherapy with either drug alone, co-treatment more successfully improved motor skill learning and coordination in N171-82Q mice, and suppressed anxiety-like behaviors in YAC128 mice. This combined treatment consistently inhibited GSK-3β and HDACs, and caused a sustained elevation in striatal as well as cortical brain-derived neurotrophic factor and heat shock protein 70. Importantly, co-treatment markedly prolonged median survival of N171-82Q mice from 31.6 to 41.6 weeks. Given that there is presently no proven treatment for HD, our results suggest that combined treatment with lithium and VPA, two mood stabilizers with a long history of safe use in humans, may have important therapeutic potential for HD patients.

Lithium-induced effects on adult hippocampal neurogenesis are topographically segregated along the dorso-ventral axis of stressed mice

Adult hippocampal neurogenesis is an important process in the regulation of cognition, stress responsivity, and sensitivity to antidepressant and mood stabiliser drugs. Increasing evidence suggests that the hippocampus is functionally divided along its axis with the ventral hippocampus (vHi) playing a preferential role in stress- and anxiety-related processes, while the dorsal hippocampus (dHi) is crucial for spatial learning and memory. However, it is currently unclear whether stress or the medications used to treat stress-related disorders, preferentially affect neurogenesis in the vHi rather than dHi. The aim of this study was to determine whether the mood stabiliser, lithium, preferentially affects cell proliferation and survival in the vHi rather than dHi under stress conditions. To this end, mice of the stress-sensitive strain, BALB/c, underwent chronic exposure to immobilisation stress plus lithium treatment (0.2% lithium-supplemented diet), and the rates of cell proliferation and survival were compared in the dHi and vHi. Lithium preferentially increased cell proliferation in the vHi under stress conditions only. This increase in cell proliferation was secondary to reductions in the survival of newly-born cells. Moreover, lithium-induced decreases in cell survival in the vHi were only observed under stress conditions. Taken together, the data suggest that the turnover of newly-born cells in response to chronic stress and lithium treatment occurs predominantly in the vHi rather than the dHi. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Behavioral and pharmacological assessment of a potential new mouse model for mania

Bipolar disorder (BPD) is a devastating long-term disease for which a significant symptom is mania. Rodent models for mania include psychostimulant-induced hyperactivity and single gene alterations, such as in the Clock or DAT genes, but there is still a pressing need for additional models. Recently, our lab isolated a line of mice, termed Madison (MSN), that exhibit behavioral characteristics that may be analogous to symptoms of mania. In this study we quantified possible traits for mania and tested the response to common anti-BPD drugs in altering the behavioral profiles observed in this strain. Relative to other mouse lines, MSN mice showed significant elevations of in-cage hyperactivity levels, significant decreases in daytime sleep, and significant increases in time swimming in the forced swim test. In terms of sexual behavior, the MSN mice showed significantly higher number of mounts and a trend toward higher time mounting. In separate studies, olanzapine and lithium (or respective controls) were administered to MSN mice for at least 2weeks and response to treatments was evaluated. Olanzapine (1mg/kg/day) significantly decreased in-cage hyperactivity and significantly increased time sleeping. Lithium (0.2-0.4% in food) significantly decreased in-cage hyperactivity. Given the behavioral phenotypes and the response to anti-BPD treatments, we propose that MSN mice may provide a possible new model for understanding the neural and genetic basis of phenotypes related to mania and for developing pharmaceutical treatments.

Antidepressant-like responses to lithium in genetically diverse mouse strains

A mood stabilizing and antidepressant response to lithium is only found in a subgroup of patients with bipolar disorder and depression. Identifying strains of mice that manifest differential behavioral responses to lithium may assist in the identification of genomic and other biologic factors that play a role in lithium responsiveness. Mouse strains were tested in the forced swim test (FST), tail suspension test (TST) and open-field test after acute and chronic systemic and intracerebroventricular (ICV) lithium treatments. Serum and brain lithium levels were measured. Three (129S6/SvEvTac, C3H/HeNHsd and C57BL/6J) of the eight inbred strains tested, and one (CD-1) of the three outbred strains, showed an antidepressant-like response in the FST following acute systemic administration of lithium. The three responsive inbred strains, as well as the DBA/2J strain, displayed antidepressant-like responses to lithium in the FST after chronic administration of lithium. However, in the TST, acute lithium resulted in an antidepressant-like effect only in C3H/HeNHsd mice. Only C57BL/6J and DBA/2J showed an antidepressant-like response to lithium in the TST after chronic administration. ICV lithium administration resulted in a similar response profile in BALB/cJ (non-responsive) and C57BL/6J (responsive) strains. Serum and brain lithium concentrations showed that behavioral results were not because of differential pharmacokinetics of lithium in individual strains, suggesting that genetic factors likely regulate these behavioral responses to lithium. Our results indicate that antidepressant-like responses to lithium in tests of antidepressant efficacy varies among genetically diverse mouse strains. These results will assist in identifying genomic factors associated with lithium responsiveness and the mechanisms of lithium action.

Altered sensitivity of cerebellar granule cells to glutamate receptor overactivation in the Cln3(Δex7/8)-knock-in mouse model of juvenile neuronal ceroid lipofuscinosis

The juvenile onset form of neuronal ceroid lipofuscinoses (JNCL) is a recessively inherited lysosomal storage disorder characterized by progressive neurodegeneration. JNCL results from mutations in the CLN3 gene that encodes a lysosomal membrane protein with unknown function. Utilizing a Cln3-knock-out mouse model of JNCL that was created on the 129S6/SvEv genetic background, we have previously demonstrated that CLN3-deficient cerebellar granule cells (CGCs) have a selectively increased sensitivity to AMPA-type glutamate receptor-mediated toxicity. Our recent findings that CGCs from 129S6/SvEv and C57BL/6J wild type (WT) mice have significant differences in glutamate receptor expression and in excitotoxic vulnerability indicated that the genetic background possibly have a strong influence on how glutamate receptor function is dysregulated in CLN3-deficient neurons. Indeed, here we show that in the Cln3(Δex7/8)-knock-in mouse model, that is on the C57BL/6J genetic background, mimics the most frequent mutation observed in JNCL patients and considered a null mutant, the sensitivity of CGCs to both AMPA- and NMDA-type glutamate receptor overactivations is altered. Cultured wild type and Cln3(Δex7/8) CGCs were equally sensitive to AMPA toxicity after 2 or 3 weeks in vitro, whereas the subunit-selective AMPA receptor agonist, CPW-399, induced significantly more cell death in mature, 3-week-old Cln3(Δex7/8) cultures. NMDA receptor-mediated toxicity changed during in vitro development: Cln3(Δex7/8) CGCs were less sensitive to high concentration of NMDA after 2 weeks in culture but became more vulnerable than their WT counterparts after 3 weeks in vitro. Abnormally altered glutamate receptor function in the cerebellum may result in motor deficits, and we confirmed that 7-week-old Cln3(Δex7/8) mice, similarly to Cln3-knock-out mice, have a motor coordination deficit as measured by an accelerating rotarod. Our results demonstrate altered glutamate receptor function in Cln3(Δex7/8) neurons and suggest that both AMPA and NMDA receptors are potential therapeutic targets in JNCL.

Phosphodiesterase 4 inhibitors enhance sexual pleasure-seeking activity in rodents

Pleasure-seeking deficits, including lack of libido, are a core feature of depression. Animal and preliminary clinical studies both suggest that phosphodiesterase 4 (PDE4) is a target for developing novel antidepressants. This study examined the potential involvement of PDE4 in the pathology of depression in both animal models and human postmortem brains. In humans, PDE4B and PDE4D levels were elevated in cingulate cortical tissue from individuals with major depressive disorder (MDD) compared to controls. Using the female urine smelling test (FUST), a recently refined method for monitoring sexual pleasure-seeking activity in mice, we found that icv infusion of selective potent PDE4 inhibitors enhanced sexual pleasure-seeking activity in male mice that underwent the learned helplessness or serotonin depletion paradigms. The infusion also increased sexual pleasure-seeking activity in naïve male mice. The results suggest that PDE4 may be a plausible contributor to the sexual pleasure-seeking deficits seen in depressed patients; inhibiting PDE4 may restore these deficits.

Synaptic plasticity in the pathophysiology and treatment of bipolar disorder

Emerging evidence suggests that synaptic plasticity is intimately involved in the pathophysiology and treatment of bipolar disorder (BPD). Under certain conditions, over-strengthened and/or weakened synapses at different circuits in the brain could disturb brain functions in parallel, causing manic-like or depressive-like behaviors in animal models. In this chapter, we summarize the regulation of synaptic plasticity by medications, psychological conditions, hormones, and neurotrophic factors, and their correlation with mood-associated animal behaviors. We conclude that increased serotonin, norepinephrine, dopamine, brain-derived neurotrophic factor (BDNF), acute corticosterone, and antidepressant treatments lead to enhanced synaptic strength in the hippocampus and also correlate with antidepressant-like behaviors. In contrast, inhibiting monoaminergic signaling, long-term stress, and pathophysiological concentrations of cytokines weakens glutamatergic synaptic strength in the hippocampus and is associated with depressive-like symptoms.

Mood disorder susceptibility gene CACNA1C modifies mood-related behaviors in mice and interacts with sex to influence behavior in mice and diagnosis in humans

BACKGROUND

Recent genome-wide association studies have associated polymorphisms in the gene CACNA1C, which codes for Ca(v)1.2, with a bipolar disorder and depression diagnosis.

METHODS

The behaviors of wild-type and Cacna1c heterozygous mice of both sexes were evaluated in a number of tests. Based upon sex differences in our mouse data, we assessed a gene × sex interaction for diagnosis of mood disorders in human subjects. Data from the National Institute of Mental Health Genetics Initiative Bipolar Disorder Consortium and the Genetics of Recurrent Early-Onset Major Depression Consortium were examined using a combined dataset that included 2021 mood disorder cases (1223 female cases) and 1840 control subjects (837 female subjects).

RESULTS

In both male and female mice, Cacna1c haploinsufficiency was associated with lower exploratory behavior, decreased response to amphetamine, and antidepressant-like behavior in the forced swim and tail suspension tests. Female, but not male, heterozygous mice displayed decreased risk-taking behavior or increased anxiety in multiple tests, greater attenuation of amphetamine-induced hyperlocomotion, decreased development of learned helplessness, and a decreased acoustic startle response, indicating a sex-specific role of Cacna1c. In humans, sex-specific genetic association was seen for two intronic single nucleotide polymorphisms, rs2370419 and rs2470411, in CACNA1C, with effects in female subjects (odds ratio = 1.64, 1.32) but not in male subjects (odds ratio = .82, .86). The interactions by sex were significant after correction for testing 190 single nucleotide polymorphisms (p = 1.4 × 10⁻⁴, 2.1 × 10⁻⁴; p(corrected) = .03, .04) and were consistent across two large datasets.

CONCLUSIONS

Our preclinical results support a role for CACNA1C in mood disorder pathophysiology, and the combination of human genetic and preclinical data support an interaction between sex and genotype.

Antidepressant-like effects of an AMPA receptor potentiator under a chronic mild stress paradigm

Enhancement of AMPA receptor (AMPAR) function has emerged as a novel strategy for treatment of depression. Nevertheless, studies on AMPAR function in chronic animal models used to predict antidepressant efficacy are surprisingly lacking. We investigated the role of AMPARs in antidepressant action in an unpredictable chronic mild stress (UCMS) model in BALB/c mice. After 3 wk of UCMS, BALB/c mice developed a number of depressive-like behaviours that were successfully prevented by fluoxetine (20 mg/kg) administration. The AMPAR potentiator LY392098 [N-2-(4-(3-thienyl)phenyl)propyl 2-propanesulfonamide] (5 mg/kg), when administered alone, functioned like classic antidepressants by reducing weight loss, fur deterioration and immobility in the tail suspension test. However, LY392098 did not restore sucrose preference and did not reduce anxiety (marble-burying) in stressed mice. In the same protocol, the AMPAR antagonist GYKI (10 mg/kg) reversed most, but not all, of the antidepressant-like actions of fluoxetine. Thus, the antidepressant-like effects of LY392098 were fully predicted by the AMPAR dependence of effects demonstrated for fluoxetine. Our results demonstrate that, in the UCMS paradigm, AMPAR activation exhibits antidepressant-like activity that relates preferentially to specific depressive-like responses and that those specific responses can be defined by their regulation by AMPAR modulation under conditions of stress.

Translational research in bipolar disorder: emerging insights from genetically based models

Bipolar disorder (BPD) is characterized by vulnerability to episodic depression and mania and spontaneous cycling. Because of marked advances in candidate-gene and genome-wide association studies, the list of risk genes for BPD is growing rapidly, creating an unprecedented opportunity to understand the pathophysiology of BPD and to develop novel therapeutics for its treatment. However, genetic findings are associated with major unresolved issues, including whether and how risk variance leads to behavioral abnormalities. Although animal studies are key to resolving these issues, consensus is needed regarding how to define and monitor phenotypes related to mania, depression and mood swing vulnerability in genetically manipulated rodents. In this study we discuss multiple facets of this challenging area, including theoretical considerations, available tests, limitations associated with rodent behavioral modeling and promising molecular-behavioral findings. These include CLOCK, glycogen synthase kinase 3beta (GSK-3beta), glutamate receptor 6 (GluR6), extracellular signal-regulated kinase-1 (ERK1), p11 (or S100A10), vesicular monoamine transporter 2 (VMAT2 or SLC18A2), glucocorticoid receptors (GRs), Bcl-2-associated athanogene-1 (BAG1) and mitochondrial DNA polymerase-gamma (POLG). Some mutant rodent strains show behavioral clusters or activity patterns that cross-species phenocopy objective/observable facets of mood syndromes, and changes in these clustered behaviors can be used as outcome measures in genetic-behavioral research in BPD.

A kinesin signaling complex mediates the ability of GSK-3beta to affect mood-associated behaviors

Lithium has been the gold standard in the treatment of bipolar disorder (BPD) for 60 y. Like lithium, glycogen synthase kinase 3 (GSK-3) inhibitors display both antimanic-like and antidepressant-like effects in some animal models. However, the molecular mechanisms of both lithium and GSK-3 inhibitors remain unclear. Here we show that the GSK-3 inhibitor AR-A014418 regulated alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA)-induced GluR1 and GluR2 internalization via phosphorylation of kinesin light chain 2 (KLC2), the key molecule of the kinesin cargo delivery system. Specifically, AMPA stimulation triggered serine phosphorylation of KLC2 and, subsequently, the dissociation of the GluR1/KLC2 protein complex. This suggests that GSK-3 phosphorylation of KLC2 led to the dissociation of AMPA-containing vesicles from the kinesin cargo system. The peptide TAT-KLCpCDK, a specific inhibitor for KLC2 phosphorylation by GSK-3beta, reduced the formation of long-term depression. Furthermore, the TAT-KLCpCDK peptide showed antimanic-like effects similar to lithium's on amphetamine-induced hyperactivity, a frequently used animal model of mania. It also induced antidepressant-like effects in the tail suspension and forced swim tests, two commonly used animal models of depression. Taken together, the results demonstrated that KLC2 is a cellular target of GSK-3beta capable of regulating synaptic plasticity, particularly AMPA receptor trafficking, as well as mood-associated behaviors in animal models. The kinesin cargo system may provide valuable novel targets for the development of new therapeutics for mood disorders.

Chronic fluoxetine treatment induces structural plasticity and selective changes in glutamate receptor subunits in the rat cerebral cortex

It has been postulated that chronic administration of antidepressant drugs induces delayed structural and molecular adaptations at glutamatergic forebrain synapses that might underlie mood improvement. To gain further insight into these changes in the cerebral cortex, rats were treated with fluoxetine (flx) for 4 weeks. These animals showed decreased anxiety and learned helplessness. N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunit levels (NR1, NR2A, NR2B, GluR1 and GluR2) were analysed in the forebrain by both western blot of homogenates and immunohistochemistry. Both methods demonstrated an upregulation of NR2A, GluR1 and GluR2 that was especially significant in the retrosplenial granular b cortex (RSGb). However, when analysing subunit content in postsynaptic densities and synaptic membranes, we found increases of NR2A and GluR2 but not GluR1. Instead, GluR1 was augmented in a microsomal fraction containing intracellular membranes. NR1 and GluR2 were co-immunoprecipitated from postsynaptic densities and synaptic membranes. In the immunoprecipitates, NR2A was increased while GluR1 was decreased supporting a change in receptor stoichiometry. The changes of subunit levels were associated with an upregulation of dendritic spine density and of large, mushroom-type spines. These molecular and structural adaptations might be involved in neuronal network stabilization following long-term flx treatment.

NMDA receptor antagonists augment antidepressant-like effects of lithium in the mouse forced swimming test

Although there is evidence of the involvement of N-methyl-D-aspartate receptors (NMDAR) in the action of lithium, its role in the antidepressant effects of lithium in a behavioural model remains unclear. In this study, we evaluated the effects of NMDAR antagonists on the antidepressant-like effects of lithium in the mouse forced swimming test. Lithium (30 and 100 mg/kg, i.p.) significantly (P < 0.01) reduced the immobility times of mice, whereas at lower doses (5 and 10 mg/kg) had no effect. NMDA antagonists ketamine (2 and 5 mg/kg, i.p.), MK-801 (0.1 and 0.25 mg/kg, i.p.) and ifenprodil (1 and 3 mg/kg, i.p.) significantly (P < 0.05) decreased the immobility time. Lower doses of ketamine (0.5 and 1 mg/kg), MK-801 (0.01 and 0.05 mg/kg) and ifenprodil (0.1 and 0.5 mg/kg) had no effect. Combined treatment of subeffective doses of lithium (10 mg/kg) and ketamine (1 mg/kg), MK-801 (0.05 mg/kg) or ifenprodil (0.5 mg/kg) robustly (P < 0.001) exerted an antidepressant-like effect. The noneffective dose of a NMDA agonist (NMDA, 75 mg/kg, i.p.) prevented the antidepressant-like effect of lithium (30 mg/kg). None of the drugs at subactive doses or in combination with lithium had significant effect on the locomotor activity in the open field test. We for the first time suggested a role for NMDAR signalling in the antidepressant-like effects of lithium, providing a new approach for treatment of depression.

Behavioral analyses of transgenic mice harboring bipolar disorder candidate genes, IMPA1 and IMPA2

The inositol depletion hypothesis proposes the inhibition of IMPase (myo-inositol monophosphatase) by lithium, a mood stabilizer, as a mechanism of lithium's efficacy. This hypothesis predicts that the upregulation of this biochemical pathway may underlie the pathophysiology of bipolar disorder. In favor of this idea, IMPA2 encoding IMPase is a candidate susceptibility gene for bipolar disorder and that the risk-conferring single nucleotide polymorphisms enhance the promoter activity of IMPA2. However, it is yet unknown whether such upregulation has a biological role in bipolar disorder. To address this issue, we generated transgenic mice for the two IMPase genes (IMPA1 and IMPA2). The expression levels of the transgene were robust in IMPA2 Tg lines, but moderate in IMPA1 Tg lines, when compared to those of endogenous proteins. The transgenic mice behaved normally under drug-naïve conditions, and did not exhibit signs for manic change when an antidepressant amitriptyline was administrated. Interestingly, the male transgenic mice for IMPA2 exhibited a lithium-resistant phenotype in the forced swim test. The current study, as a whole, did not support a substantial role of the upregulation of IMPase in bipolar disorder, although the lithium-insensitivity trait seen in IMPA2 transgenic mice might represent some aspect relevant to the inositol depletion hypothesis.

Strain-specific battery of tests for domains of mania: effects of valproate, lithium and imipramine

The lack of efficient animal models for bipolar disorder (BPD), especially for the manic pole, is a major factor hindering the research of its pathophysiology and the development of improved drug treatments. The present study was designed to identify an appropriate mouse strain for modeling some behavioral domains of mania and to evaluate the effects of drugs using this strain. The study compared the behavior of four strains: Black Swiss, C57Bl/6, CBA/J and A/J mice in a battery of tests that included spontaneous activity; sweet solution preference; light/dark box; resident-intruder; forced-swim and amphetamine-induced hyperactivity. Based on the 'manic-like' behavior demonstrated by the Black Swiss strain, the study evaluated the effects of the mood stabilizers valproate and lithium and of the antidepressant imipramine in the same tests using this strain. Results indicated that lithium and valproate attenuate the 'manic-like' behavior of Black Swiss mice whereas imipramine had no effects. These findings suggest that Black Swiss mice might be a good choice for modeling several domains of mania and distinguishing the effects of drugs on these specific domains. However, the relevance of the behavioral phenotype of Black Swiss mice to the biology of BPD is unknown at this time and future studies will investigate molecular differences between Black Swiss mice and other strains and asess the interaction between strain and mood stabilizing treatment.

Reduced expression of glutamate transporters vGluT1, EAAT2 and EAAT4 in learned helpless rats, an animal model of depression

BACKGROUND

It has been widely accepted that glial pathology and disturbed synaptic transmission contribute to the neurobiology of depression. Apart from monoaminergic alterations, an influence of glutamatergic signal transduction has been reported. Therefore, gene expression of glutamate transporters that strictly control synaptic glutamate concentrations have to be assessed in animal models of stress and depression.

METHODS

We performed in situ-hybridizations in learned helplessness rats, a well established animal model of depression, to assess vGluT1 and EAAT1-4. Sprague-Dawley rats of two inbred lines were tested for helpless behavior and grouped into three cohorts according to the number of failures to stop foot shock currents by lever pressing.

RESULTS

Helpless animals showed a significantly suppressed expression of the glial glutamate transporter EAAT2 (rodent nomenclature GLT1) in hippocampus and cerebral cortex compared to littermates with low failure rate and not helpless animals. This finding was validated on protein level using immunohistochemistry. Additionally, expression levels of EAAT4 and the vesicular transporter vGluT1 were reduced in helpless animals. In contrast, the transcript levels of EAAT1 (GLAST) and EAAT3 (EAAC1) were not significantly altered.

CONCLUSIONS

These results strongly suggest reduced astroglial glutamate uptake and implicate increased glutamate levels in learned helplessness. The findings are in concert with antidepressant effects of NMDA-receptor antagonists and the hypotheses that impaired astroglial functions contribute to the pathogenesis of affective disorders.

Possible Role of Platelet GluR1 Receptors in Comorbid Depression and Cardiovascular Disease

The exact nature of the comorbidity between cardiovascular disease (CVD) and major depressive disorder (MDD) is poorly understood. The proposed mechanisms include various biochemical and molecular pathways as well as health behaviors such as physical inactivity. One possible link between MDD and CVD is increased platelet activity and blood viscosity. Recently, it was discovered that platelets express functional subtype of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, for example, glutamate receptor 1 (GluR1). Here, I propose that this type of AMPA receptor could play a role in comorbid MDD and CVD, and antidepressants may interfere with platelet activation via direct or indirect effects on platelet GluR1 phosphorylation. Testing this hypothesis could provide a novel view on the pathobiological mechanisms of comorbid MDD and CVD. With respect to the recently discovered role of AMPA receptors in regulating platelet activation and thrombosis, it appears that the information about the putative effects of psychoactive AMPA-modifying drugs on platelet AMPA receptors would be critical in evaluating the putative effects of such drugs on CVD.

Melatonin as a potential therapeutic agent in psychiatric illness

The aim of this review was to summarize the potential use of melatonin in the treatment of mental disorders, specifically bipolar disorders, depression, and schizophrenia. To date, melatonin has been most commonly used in psychiatry because of its hypnotic, rhythm resynchronizing, and antioxidant actions. Here, we examine other properties of the melatonin including its anti-inflammatory, antinociceptive, anxiolytic, and drug detoxification actions as well as its protective effects against neural loss. The brain is an intricate sensory and motor organ which receives information from both the external and internal environments. It transduces information into complex chemical and electrical signals which are transmitted throughout the central nervous system (CNS) and the organism. The pathogenesis of mental disorders remains ambiguous and neuroinflammation has been proposed as a causative agent. We consider the potential contributions of melatonin as therapeutic agent in CNS and during neuroinflammation in mental disorders.

Chronic fluoxetine treatment increases expression of synaptic proteins in the hippocampus of the ovariectomized rat: role of BDNF signalling

Antidepressant drugs have been suggested to regulate synaptic transmission and structure. We hypothesised that antidepressant-induced changes in synapses and their associated proteins might become more apparent if they were measured under conditions of reduced synapse density. Therefore, in the present study, we examined whether chronic treatment with the antidepressant, fluoxetine alters expression of synaptic proteins in the hippocampus of rodents that underwent ovariectomy, a procedure which reportedly decreases synapse density in the CA1 region of the rat hippocampus. Using Western blotting, we measured changes in hippocampal expression of proteins associated with synapse structure, strength and activity namely, postsynaptic density protein 95 (PSD-95), the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA-R) subunit GluR1 and phosphosynapsin (Ser9), respectively. We found that fluoxetine treatment increased expression of phosphosynapsin, PSD-95 and synaptic GluR1 (but not total GluR1) in the hippocampus of ovariectomized but not sham rats. Since BDNF and signalling at its receptor, TrkB, can mediate behavioural responses to antidepressants and induce neuronal plasticity, we investigated the contribution of TrkB signalling to fluoxetine-induced changes in synaptic protein expression by using a transgenic mouse model overexpressing a truncated form of the TrkB receptor (TrkB.T1). Fluoxetine produced a small but significant increase in hippocampal PSD-95 in ovariectomized wildtype mice but not in ovariectomized TrkB.T1 mice or sham mice. In contrast to rats, fluoxetine did not alter expression of synaptic GluR1 and did not reverse ovariectomy-induced decreases in hippocampal phosphosynapsin in either genotype. Taken together, these results suggest that chronic fluoxetine treatment can increase synaptic protein expression in the hippocampus and at least some of these effects require TrkB signalling. Moreover, these effects were only observed in ovariectomized animals, thus suggesting that fluoxetine-induced increases in synaptic protein levels might only occur or become detectable when hippocampal synaptic connectivity is perturbed.

Dual- and triple-acting agents for treating core and co-morbid symptoms of major depression: novel concepts, new drugs

The past decade of efforts to find improved treatment for major depression has been dominated by genome-driven programs of rational drug discovery directed toward highly selective ligands for nonmonoaminergic agents. Selective drugs may prove beneficial for specific symptoms, for certain patient subpopulations, or both. However, network analyses of the brain and its dysfunction suggest that agents with multiple and complementary modes of action are more likely to show broad-based efficacy against core and comorbid symptoms of depression. Strategies for improved multitarget exploitation of monoaminergic mechanisms include triple inhibitors of dopamine, serotonin (5-HT) and noradrenaline reuptake, and drugs interfering with feedback actions of monoamines at inhibitory 5-HT(1A), 5-HT(1B) and possibly 5-HT(5A) and 5-HT(7) receptors. Specific subsets of postsynaptic 5-HT receptors mediating antidepressant actions are under study (e.g., 5-HT(4) and 5-HT(6)). Association of a clinically characterized antidepressant mechanism with a nonmonoaminergic component of activity is an attractive strategy. For example, agomelatine (a melatonin agonist/5-HT(2C) antagonist) has clinically proven activity in major depression. Dual neurokinin(1) antagonists/5-HT reuptake inhibitors (SRIs) and melanocortin(4) antagonists/SRIs should display advantages over their selective counterparts, and histamine H(3) antagonists/SRIs, GABA(B) antagonists/SRIs, glutamatergic/SRIs, and cholinergic agents/SRIs may counter the compromised cognitive function of depression. Finally, drugs that suppress 5-HT reuptake and blunt hypothalamo-pituitary-adrenocorticotrophic axis overdrive, or that act at intracellular proteins such as GSK-3beta, may abrogate the negative effects of chronic stress on mood and neuronal integrity. This review discusses the discovery and development of dual- and triple-acting antidepressants, focusing on novel concepts and new drugs disclosed over the last 2 to 3 years.

Lithium and cognitive enhancement: leave it or take it?

RATIONALE

Lithium is established as an effective treatment of acute mania, bipolar and unipolar depression and as prophylaxis against bipolar disorder. Accumulating evidence is also delineating a neuroprotective and neurotrophic role for lithium. However, its primary effects on cognitive functioning remain ambiguous.

OBJECTIVES

The aim of this paper is to review and combine the relevant translational studies, focusing on the putative cognitive enhancement properties of lithium, specifically on learning, memory, and attention.

DISCUSSION

These properties are also discussed in reference to research demonstrating a protective action of lithium against cognitive deficits induced by various challenges to the nervous system, such as stress, trauma, neurodegenerative disorders, and psychiatric disorders.

CONCLUSIONS

It is suggested on the basis of the evidence that the cognitive effects of lithium are best expressed and should, therefore, be sought under conditions of functional or biological challenge to the nervous system.

Glutamate Neurotransmission in Psychotic Disorders and Substance Abuse

Psychotropic substance abuse and addiction are very prevalent among individuals with major psychiatric disorders world wide. However, this significant association is poorly explained. The dopaminergic circuits have been implemented in addiction as well as in schizophrenia. Recently the important role of glutamatergic neurotransmission has gained attention and current theoretical models of psychosis and substance abuse support the role of interactions between glutamate and other neurotransmitters in the patho-physiology of both disorders. However, the identification of the underlying genetic risk factors remains challenging and not a single genomic variant has been identified with certainty, possibly due to important limitations of the methods used. Clinical trials with glutamatergic neurotransmission modulators, even though still controversial, support the role of glutamate in psychosis and justify further research.

Antidepressive-like effects of rapamycin in animal models: Implications for mTOR inhibition as a new target for treatment of affective disorders

Lithium, the prototypic mood stabilizer, was recently demonstrated to enhance autophagy in cells. Recent hypotheses regarding the source of therapeutic effects of lithium as well as other mood stabilizers and antidepressants suggest that they may stem from increased neuroprotection, cellular plasticity and resilience. Hence it is clearly a possibility that enhanced autophagy may be involved in the therapeutic action by contributing to increased cellular resilience. A well-documented mechanism to induce autophagy is by inhibition of mTOR, a negative modulator of autophagy and rapamycin (sirolimus) is a commonly used inhibitor of mTOR. Accordingly, the present study was designed to evaluate the effects of rapamycin in animal models of antidepressant activity. A dose-response experiment in the mice forced swim test was performed and followed by additional testing of mice and rats in an open field, the forced swim test and the tail suspension test. Results show that sub-chronic, but not acute, administration of rapamycin doses of 10mg/kg and above, have an antidepressant-like effect in both mice and rats and in both the forced swim and the tail suspension tests with no effects on the amount or distribution of activity in the open field. Whereas it is tempting to conclude that the antidepressant-like effects are related to mTOR inhibition, they may also be the consequences of interactions with other intracellular pathways. Additional studies are now planned to further explore the behavioral range of rapamycin's effects as well as the biological mechanisms underlying these effects.

Generation and behavioral characterization of beta-catenin forebrain-specific conditional knock-out mice

The canonical Wnt pathway and beta-catenin have been implicated in the pathophysiology of mood disorders. We generated forebrain-specific CRE-mediated conditional beta-catenin knock-out mice to begin exploring the behavioral implications of decreased Wnt pathway signaling in the central nervous system. In situ hybridization revealed a progressive knock-out of beta-catenin that began between 2 and 4 weeks of age, and by 12 weeks resulted in considerably decreased beta-catenin expression in regions of the forebrain, including the frontal cortex, hippocampus, and striatum. A significant decrease in protein levels of beta-catenin in these brain regions was observed by Western blot. Behavioral characterization of these mice in several tests (including the forced swim test, tail suspension test (TST), learned helplessness, response and sensitization to stimulants, and light/dark box among other tests) revealed relatively circumscribed alterations. In the TST, knock-out mice spent significantly less time struggling (a depression-like phenotype). However, knock-out mice did not differ from their wild-type littermates in the other behavioral tests of mood-related or anxiety-related behaviors. These results suggest that a 60-70% beta-catenin reduction in circumscribed brain regions is only capable of inducing subtle behavioral changes. Alternatively, regulating beta-catenin may modulate drug effects rather than being a model of mood disorder pathophysiology per se.